In radar and communications systems it is sometimes important to process signals having a desired range delay while rejecting signals having an undesired range delay. The out-of-range rejection (ORR) performance is a measure of a system's ability to accomplish this requirement. Pseudorandom noise (PN) coded systems are often used to achieve acceptable ORR performance while maintaining low peak transmit power requirements.
Some PN coded systems receive a biphase modulated signal, the signal having been modulated with one of two phase states responsive to the logic state of a maximal length binary code sequence, with the relative phase difference between the two phase states being approximately 180 degrees. The properties of a maximal length binary code sequence are well known to those of ordinary skill in the art and include statistical properties which closely approximate a randomly generated binary code sequence in which each code state is equally likely. However, all maximal length binary code sequences are periodic and have an odd code length of L bits with L=2.sup.N -1, where N is a positive integer. Furthermore, in any complete period, the number of occurrences of each code state differ by exactly one. Consequently, the ORR performance in PN coded systems is typically limited to 10 log (1/L.sup.2).
Many PN coded systems use analog components in the receive path to demodulate the received biphase modulated signal in a range correlation process. In such PN systems, the ORR performance is limited by imperfections in the analog components due in part to manufacturing tolerances. The imperfections are different in the analog components used to biphase modulate the transmitted signal than in the analog components used to demodulate the biphase modulated received signal. In an attempt to minimize these differences, some PN coded systems implement a biphase modulator used to demodulate the biphase modulated received signal. This biphase modulator is nearly identical to the biphase modulator used to biphase modulate the transmitted signal. However, component imperfections and differences persist between the biphase modulator in the transmit path and the biphase modulator(s) in the receive path(s), and these differences limit a more robust optimization of the ORR performance.
What is needed to optimize the ORR performance is a digital correlation technique that includes a method and apparatus for compensating imperfections in the biphase modulator in the transmit path and a digital correlator incorporating the compensating means to replace the biphase modulator(s) in the receive path(s). In PN-coded systems requiring multiple channels, with each channel responsive to a signal having a different range delay, what is needed is a digital correlation technique which reduces the amount of physical hardware required to partition the target detection space, thereby reducing cost and packaging size.
What is needed in PN-coded systems that perform range correlation using digital circuitry is a method and apparatus for performing the range correlation task and the spectral processing task concurrently, thereby reducing the size, complexity, and cost of PN coded systems, particularly in applications requiring a large number of range resolution cells. In a PN-coded system using digital Fourier Transform spectral processing, what is needed is a means for independently setting the attributes of an optimal number of spectral processing channels to provide fine Doppler frequency resolution at the lower end of the frequency spectrum where longer dwell times are acceptable and to provide for short dwell times at the upper end of the frequency spectrum where coarser Doppler frequency resolution is acceptable.